The present invention relates to a graphic input device and, more particularly, to a graphic input device of the type using conductor loop groups orthogonally arranged on an input surface of a tablet and a stylus or pen having a winding therein. This type of device senses a location where the winding in the stylus or pen is electromagnetically coupled with the conductor loop groups so as to generate an electrical signal which is representative of coordinates of the stylus or pen input position on the tablet surface.
To promote the accuracy of the operation of a graphic input device of the type described, I have described an arrangement which causes electromagnetic coupling to occur between conductor loop groups on an input surface, or a writing surface, and a winding in a stylus or pen. Either the conductor loop groups or the winding is driven by a high frequency carrier, as disclosed in U.S. patent Ser. No. 654,803 now U.S. Pat. No. 4,568,799.
The problem with the prior art graphic input device of the kind described in U.S. patent Ser. No. 654,803 is that, in the case of envelope detection, detection errors are apt to be introduced due to inclinations of the stylus or pen. These errors cannot be eliminated unless the inclinations of the stylus or pen are confined into an impractically narrow range. In the case of synchronous detection, the size of the circuit becomes undesirably large, adding to the dimensions and cost of the whole device.